I'm at the tail end of a 350 5.7l rebuild for my 1990 g20 van - Long story short is i just checked the head gaskets i bought and it says the latest compatible year is 1985 / i ordered through a rep a summit racing, gave him the the van info he filled cart and this is what came... i didnt look them over so that's my fault. these are for piston bored over 30- so I can't just go get a new set at a parts store -

I compared the new gaskets to another new set that are standard bore (obv i cant use them) and the ones that came off the block.
The main differences between them is the wider doubles holes (center top) and a few smaller holes on the lower part.
Widening out the small ones would be no problem but will everything be fine with the top holes like they are? Please see pictures.
(i tired to clearly show the comparisons: the new .30 over bore Fel-Pros have blue lines, the ones i am using to compare are Lead)

I know it's smarter to just get the right ones but long story short I'm having a tough time finding the time due to work and i want (kind of NEED) to get this buttoned up tomorrow.
So how much would this mod matter overall?

 

Most gaskets that I have seen with reduced coolant port diameters are done on purpose that way. It is to restrict flow of coolant in that area but still allow air/steam pockets to get out. I wouldn't open them up.